The present invention is directed to an improved process for the production of polyester resin having a high intrinsic viscosity.
The catalysts normally used in polycondensation of aromatic polyester resins in the molten state are in general compounds of antimony (principally, antimony oxide and antimony triacetate). Catalysts based on germanium oxide are also usable but only in certain cases, given the high cost of the catalyst.
Titanium compounds (in particular titanium alkoxides) have also been proposed as catalysts. These catalysts have a high activity, but lead to the formation of a polymer with a yellowish colouration and further, have problems of instability due to hydrolysis during synthesis of PET from terephthalic acid. The kinetics of polycondensation of the resin to the solid state is moreover detrimentally affected by the presence of titanium compounds. Because of these disadvantages, titanium catalysts have not in practice found to have an application.
Currently, the tendency of the market and the authorities competent for safeguarding the environment is to require ever more insistently a PET having a low content of residual metal catalysts. It is not, however, in practice possible to reduce the quantity of antimony catalysts because their activity is not very high.
The use of titanium catalysts is not satisfactory because of their low activity in the solid state polycondensation.
A necessity therefore exists to have available inexpensive catalysts other than those of antimony, which will not be a health hazard and which will provide good catalytic activity without presenting possible problems of colouration of the polymer.
Recently titanium dioxide and silica in the ratio Ti:Si of 9:1, and tetraisopropyl (dioctyl) titanate phosphite have been proposed as hydrolysis resistant catalysts having few problems with yellowing when compared to titanium alkoxides. The activity of these catalysts (expressed as ppm by weight of Ti/kg polymer) is very much higher than that obtainable with antimony oxide or triacetate.
These catalysts, however, also have the serious disadvantage in that their use is, in practice, precluded due to the low kinetics when they are employed for the solid state polycondensation of the resin.
With respect to antimony catalysts, in the case of PET, the kinetics of solid state polycondensation are about 50% less (for the same conditions).